Inverter behind inverter

[Pocster]

Hypothetical question !

 

An inverter connected to grid . DNO will allow no further inverters I.e max output reached .

I believe you can connect another inverter ( Slave ) to the original ( master ) . DNO happy with this as no effective change in what’s connected to the grid .

Master inverter can output 3.6kwh I.e draw from grid 

Can slave ( assuming same spec as master ) do the same ?

Appreciate limitation will be export which all goes through master inverter .

Can both draw from the grid independently?

[SteamyTea]

Steca used to do a system 2kW system that you can add extra 2 kW slave units to.

And they kicked in at a low voltage, 80V I think.

You may be able to do things with micro inverters as well.

[Pocster]

24 minutes ago, SteamyTea said:

Steca used to do a system 2kW system that you can add extra 2 kW slave units to.

And they kicked in at a low voltage, 80V I think.

You may be able to do things with micro inverters as well.

Sure . But does that mean each slave inverter can draw from the grid upto its max ? 

[Alan Ambrose]

>>> Master inverter can output 3.6kwh I.e draw from grid 

 

That doesn't make much sense. An inverter takes power ('draws') from your PV panels and generates 230V power. You may use that to power your house and/or send power back to the grid. Now some inverters control battery modules too and can then 'draw from the grid' and 'charge a battery'.

 

Is your objective to get round the DNO export limits? Nothing wrong with that, but maybe you can clarify?

Edited June 27, 2023 by Alan Ambrose

[Pocster]

5 minutes ago, Alan Ambrose said:

>>> Master inverter can output 3.6kwh I.e draw from grid 

 

That doesn't make much sense. An inverter takes power ('draws') from your PV panels and generates 230V power. You may use that to power your house and/or send power back to the grid. Now some inverters control battery modules too and can then 'draw from the grid' and 'charge a battery'.

 

Is your objective to get round the DNO export limits? Nothing wrong with that, but maybe you can clarify?

Was incorrectly phrased .

 

My objective is to charge 2 x 10kw batteries simultaneously at 3.6kwh - but with only 1 inverter grid connected . Hence this so called inverter behind inverter theory .

Edited June 27, 2023 by pocster

[SteamyTea]

20 minutes ago, pocster said:

My objective is to charge 2 x 10kw batteries simultaneously at 3.6kwh

Stop being a (expletive deleted).

10 kWh at up to 3.6 kW.

 

I think what you are after is a smarter charge controller, not a new inverter.

[Pocster]

8 minutes ago, SteamyTea said:

Stop being a (expletive deleted).

10 kWh at up to 3.6 kW.

 

I think what you are after is a smarter charge controller, not a new inverter.

Nope !

If I could have another inverter connected directly to the grid then no issue . As I can’t I’m trying to draw more from the grid to allow simultaneous charging . Tbh I see it as no different as sticking another kettle on …. 

[SteamyTea]

5 minutes ago, pocster said:

If I could have another inverter connected directly to the grid then no issue

How is an inverter going to charge the battery, or are you talking about hydrid ones that can manage your battery charging and PV?

[Alan Ambrose]

>>> My objective is to charge 2 x 10kw batteries simultaneously at 3.6kwh - but with only 1 inverter grid connected . Hence this so called inverter behind inverter theory.

 

OK you have single phase and therefore a 3.6kW / 16A export limit. You probably have a 100A main fuse which is primarily a protection against 100A draw down / import e.g. caused by a bad short. Otherwise you can take / draw / import up to 100A i.e. 23 kW.

 

Note that the battery charging function is really a separate function compared to the power inverter function (i.e. taking the battery output and generating 230V AC.) - although the functions often have basic coordination.

 

In principle, you can charge your batteries as fast as the battery pack / inverter allows - usually faster charging means shorter life. So, you could charge 20kWh of batteries at say 5kW and it will take 4 hours (i.e. 20/5=4) or at 2kW and it will take 10 hours (20/2=10).

 

Now, a typical 48V battery (e.g. Pylontech) has a recommended charge/discharge current of 25A suggesting the fastest charging/discharging you should be doing is 2 hours to full charge/discharge. That's the fastest - you can charge/discharge as slow as you like.

 

In short, yeah, I agree, you just need a reasonably smart battery charge controller. Keep the max inverter output to 3.6kW to keep your DNO happy.

 

Maybe you need to catch up on some PV/inverter operation details - it can be a bit involved as you can see. The relationship between power, voltage and current is a good start, as is the difference between kW, kWh and kWp.

 

Or just decide what you want to do at a high level, and then ask an installer.

[Pocster]

2 minutes ago, Alan Ambrose said:

>>> My objective is to charge 2 x 10kw batteries simultaneously at 3.6kwh - but with only 1 inverter grid connected . Hence this so called inverter behind inverter theory.

 

OK you have single phase and therefore a 3.6kW / 16A export limit. You probably have a 100A main fuse which is primarily a protection against 100A draw down / import e.g. caused by a bad short. Otherwise you can take / draw / import up to 100A i.e. 23 kW.

 

Note that the battery charging function is really a separate function compared to the power inverter function (i.e. taking the battery output and generating 230V AC.) - although the functions often have basic coordination.

 

In principle, you can charge your batteries as fast as the battery pack / inverter allows - usually faster charging means shorter life. So, you could charge 20kWh of batteries at say 5kW and it will take 4 hours (i.e. 20/5=4) or at 2kW and it will take 10 hours (20/2=10).

 

Now, a typical 48V battery (e.g. Pylontech) has a recommended charge/discharge current of 25A suggesting the fastest charging/discharging you should be doing is 2 hours to full charge/discharge. That's the fastest - you can charge/discharge as slow as you like.

 

In short, yeah, I agree, you just need a reasonably smart battery charge controller. Keep the max inverter output to 3.6kW to keep your DNO happy.

 

Maybe you need to catch up on some PV/inverter operation details - it can be a bit involved as you can see. The relationship between power, voltage and current is a good start, as is the difference between kW, kWh and kWp.

 

Or just decide what you want to do at a high level, and then ask an installer.

Thanks for that . I tend to ask DNO first for advice . They only help so far and then state they aren’t installers . My experience with installers varies - at the current inflated prices they charge I’d like to know what to ask - as sometimes you get rather different answers .

[Alan Ambrose]

>>> My experience with installers varies - at the current inflated prices they charge I’d like to know what to ask - as sometimes you get rather different answers .

 

Yeah understand. An alternative is just to alight on a likely inverter/charge controller supplier and ask them (or read their specs in detail) to see whether their 3.6kW inverter boxes can also handle charge/discharge on 20 kWh of batteries.

[Pocster]

5 minutes ago, Alan Ambrose said:

>>> My experience with installers varies - at the current inflated prices they charge I’d like to know what to ask - as sometimes you get rather different answers .

 

Yeah understand. An alternative is just to alight on a likely inverter/charge controller supplier and ask them (or read their specs in detail) to see whether their 3.6kW inverter boxes can also handle charge/discharge on 20 kWh of batteries.

But isn’t that the issue ? . I want to charge both at 3.6kw simultaneously I.e draw 7.2kw - but only have 1 master inverter .

[HughF]

So you want a secondary inverter that has its export function disabled. I know that outback power systems products can stack in a master/space configuration, and they don’t export.

 

That doesn’t help you though as you won’t be using their products. What you need is a battery charger, not an inverter. If you’re charging pylontech then look for an off-grid inverter that lacks pv input and talks the same can us as your batteries. Victron multiplus might be a good start.

[joth]

18 hours ago, pocster said:

Was incorrectly phrased .

 

My objective is to charge 2 x 10kw batteries simultaneously at 3.6kwh - but with only 1 inverter grid connected . Hence this so called inverter behind inverter theory .

To clarify, thes are not abstract 10kWh batteries, they are specifically your Tesla PW and solaredge energybank, yes?

So one is ac coupled and the other DC coupled. If you have export limit on the SE inverter it should already do exactly what you want: charge the SE battery direct from the panels and and excess (over the 5kW limit it will charge at) will go to AC and charge the PW.

 

Are you actually asking how to add more panels to this system? 

 

[Pocster]

4 minutes ago, joth said:

To clarify, thes are not abstract 10kWh batteries, they are specifically your Tesla PW and solaredge energybank, yes?

So one is ac coupled and the other DC coupled. If you have export limit on the SE inverter it should already do exactly what you want: charge the SE battery direct from the panels and and excess (over the 5kW limit it will charge at) will go to AC and charge the PW.

 

Are you actually asking how to add more panels to this system? 

 

Ignore pw . It’s 2 SE batteries I wish to charge . No extra pv .

[joth]

45 minutes ago, pocster said:

Ignore pw . It’s 2 SE batteries I wish to charge . No extra pv .

They're DC coupled so you can connect as many as you want to the SE inverter, no dno limit relevant, no notification required.

 

 

 

Talk to a competent SE installer 

[HughF]

3 minutes ago, joth said:

They're DC coupled so you can connect as many as you want to the SE inverter, no dno limit relevant, no notification required.

 

 

 

Talk to a competent SE installer 

Ah, well that makes things a piece of cake then. Just needs another charger hooked up to the DC bus.

[joth]

2 minutes ago, HughF said:

Ah, well that makes things a piece of cake then. Just needs another charger hooked up to the DC bus.

The SE battery has integrated charger so you literally just need a second battery hooked up to the inverter.

Caveat: if you decide to charge batteries from the grid (e.g. overnight cheap) it has to come from the (master) inverter so there will be some whole system charge rate limit, probably 5kW. But that's still enough to completely fill 2x 10kWh batts in Octopus Go cheap window. 

[HughF]

26 minutes ago, joth said:

The SE battery has integrated charger so you literally just need a second battery hooked up to the inverter.

Caveat: if you decide to charge batteries from the grid (e.g. overnight cheap) it has to come from the (master) inverter so there will be some whole system charge rate limit, probably 5kW. But that's still enough to completely fill 2x 10kWh batts in Octopus Go cheap window. 

Fancy…. I’m used to batteries being batteries….

[Pocster]

4 hours ago, joth said:

The SE battery has integrated charger so you literally just need a second battery hooked up to the inverter.

Caveat: if you decide to charge batteries from the grid (e.g. overnight cheap) it has to come from the (master) inverter so there will be some whole system charge rate limit, probably 5kW. But that's still enough to completely fill 2x 10kWh batts in Octopus Go cheap window. 

Now that’s exactly what I was trying to ask ! . I need to charge both within my 3 hour off peak window . What ‘spec’ do I look at on the charger for if it supports that ? . My inverter has a 3.68 limit hence the 2nd inverter ‘theory ‘ ….

[joth]

59 minutes ago, pocster said:

Now that’s exactly what I was trying to ask ! . I need to charge both within my 3 hour off peak window . What ‘spec’ do I look at on the charger for if it supports that ? . My inverter has a 3.68 limit hence the 2nd inverter ‘theory ‘ ….

 

3.68kW is the DC to AC rating. How do you know this is the AC to DC limit? Mine charges at 5kW (albeit on an 8kW inverter).

 

This is entirely unrelated to dno or export, you need to ask SE. Probably best to raise a support ticket with them saying you need to charge two of their batteries from grid at 7kW so that both of them (20kWh total capacity) can be filled in a three hour window.

 

[Pocster]

31 minutes ago, joth said:

3.68kW is the DC to AC rating. How do you know this is the AC to DC limit? Mine charges at 5kW (albeit on an 8kW inverter).

The SE will charge off pv at 5kw if enough sunny sunshine . When I charge from grid it’s only 3.68kw so I *assume* an inverter restriction .

SE support ranges from almost zero to a bit above zero 🫤

[joth]

You maybe on the wrong bandwagon.

I suggest you give me your current SE battery and swap to a more bespoke/flexible manufacturer for your niche requirements 

[SteamyTea]

14 hours ago, joth said:

You maybe on the wrong bandwagon.

I suggest you give me your current SE battery and swap to a more bespoke/flexible manufacturer for your niche requirements 

He has already promised it to me.

[joth]

17 minutes ago, SteamyTea said:

He has already promised it to me.

But you know his promises are worth no more than the stain riddled keyboard they were typed on.